A long-standing problem in the refrigeration arts is the “hunting” phenomenon, which is the excessive cycling of thermostatic control valves in response to transient changes in the refrigeration system. A refrigeration system generally includes a thermostatic expansion valve and a thermal sensing bulb. The thermal sensing valve is located in a position selected to provide feedback information about the thermal condition of the system which is used by the thermal expansion valve to make changes in refrigerant flow to effect changes in the thermal condition of the system. In general, a thermal sensing bulb communicates with the thermostatic expansion valve by means of a working fluid which expands or contracts with temperature changes experienced by the thermal sensing bulb and thereby affects operation of the thermostatic expansion valve. The hunting phenomenon occurs when the working fluid is, in effect, overly sensitive, resulting in rapidly fluctuating changes being communicated to the thermostatic expansion valve. In the prior art, many attempts have been made to reduce the sensitivity of the working fluid while maintaining adequate thermal control. In general, these attempts have included the use of materials that form a ballast or buffer generally located in the thermal sensing bulb, to attenuate rapid changes in the working fluid due to transient changes in the thermal condition of the system.
The assignee of the present invention has used with some success a thermostatic expansion valve and a thermal sensing bulb in which the thermal sensing bulb is provided with a single block of a material normally used as a thermal insulating board in sheet form. The sheet material is described in U.S. Pat. No. 4,128,434, the disclosure of which is incorporated herein by reference. The '434 patent describes the manufacture of the material in large sheet form (e.g., 4′×8{tilde over (′)} 122 cm.×244 cm.) for the construction industry, in which the material is used as a thermal insulation material, e.g., in walls, floors or ceilings of structures. In the assignee's previous thermal sensing bulbs, single blocks of this material are cut from such boards and one such block is inserted as a single piece into the thermal sensing bulb and forms the ballast. The block of thermal insulation material acts as a ballast for the working fluid, when the bulb is operatively attached to an appropriate thermostatic expansion valve. The material in the '434 patent is described as a thermal insulation body formed of a tobermorite calcium silicate reinforced with wollastonite. The '434 patent discloses various methods for making the sheet material. Such sheet material is available commercially under the trademark MARINITE® from BNZ Materials, Inc., Littleton, Colo.
The success had with this system has been tempered by the recurring and previously unsolved problems of variations in the material both from sheet to sheet and within a single sheet and of variations between the single blocks cut from such sheets. That is, some sheets have different thicknesses within a single sheet and compared to other, ostensibly identical sheets. Other sheets have variations in the structure of the material itself on either the macro- or micro-scale, such as differences in porosity, hardness, durability, etc. Variations also arise from differences in cutting of the individual blocks of insulation material. These variations, while not detrimental to the thermal insulation function for which the panels were designed, have resulted in problems of non-uniform operation between otherwise identical thermal expansion valves including thermal sensing bulbs using this material as ballast. Such variation in operation is undesirable for a variety of reasons, known in the art.
Other materials have been suggested for use as the ballast material in such thermal sensing bulbs used with thermostatic expansion valves. Activated carbon is one such material, sought for its porosity and adsorption characteristics. The activated carbon is used due to its controllable pore size for controlling adsorption of the working fluid. This material has its own problems, including small particle sizes, formation of fines due to breakage of the particles, and changes in adsorption due to contaminants in the working fluid. These problems manifest themselves in poor performance or performance varying with time as the adsorption characteristics change.
Thus, there is a continuing and unmet need for improved materials for use in thermal sensing units for use with expansion valves such as thermostatic expansion valves.